1. Field of the Invention
Embodiments of the present invention are directed to cerium-doped, garnet-based phosphors prepared by liquid mixing methods.
2. Description of the Related Art
The yttrium aluminate garnet phosphor known as YAG:Ce (when activated by trivalent cerium) is a well-known phosphor used in the so-called “white LED” commercial market. In comparison with phosphors based on silicates, sulphates, nitridosilicates, and oxonitridosilicates, YAG has a relatively high absorption efficiency of blue colored excitation radiation, high quantum efficiency (QE greater than about 90 percent), good stability in a high temperature and high humidity environment, and a broad emission spectrum. However, YAG's emission intensity decreases when the wavelength of the excitation radiation is reduced to a level below about 460 nm.
In the art, YAG phosphors are commonly prepared at high temperatures (greater than about 1600° C.) via a solid-state reaction method. Due to insufficient mixing and the low reactivity of the raw materials, several intermediate phases such as Y4Al2O9 (YAM) and YAlO3 (YAP) may be easily incorporated into the product compositions. Additionally, the particle size(s) of the resultant phosphor is not uniform, and typically displays a broad distribution ranges from about 1 to more than 10 micrometers when, for example, the average size (D50) in the distribution is located at about 5 micrometers.
In comparison with the solid-state-reaction method, a co-precipitation method has the advantages of preparing a substantially pure YAG phase at relatively low temperatures with a narrow distribution of the particle size. In the meantime, the luminescent properties of the co-precipitated phosphors are as good as, or perhaps even better, than the properties of those phosphors prepared by a solid-state-reaction technique.
What is needed is an improvement of YAG's emission intensity when the emission peak is centered at wavelength of from about 540 nm to 560 nm when excited by a blue diode having an emission peak wavelength at about 445 nm to 455 nm. This will produce high brightness white LEDs. To further improve the external conversion emission from a blue diode to a white diode, the scattering loss needs to be further minimized by reducing the phosphor particle size below the emission wavelength, preferably less than 400 nm.